This invention is concerned with direct-current to direct-current (DC-DC) power converters. More particularly, it is concerned with providing voltage regulation circuitry to two or more outputs using a single feedback or control loop. This circuitry is combined with a buck-flyback converter using synchronous rectification techniques to provide highly efficient power conversion and regulation functions.
There are increasing demands for power conversion circuits to operate with ever increasing efficiency due to the proliferation of portable electronic devices. In most instances, these devices are intended to be primarily driven by battery power and it is highly desirous for the battery to last as long as possible. As such, the operational voltages for various electronic devices (such as microprocessors and memory) continue to be driven to lower and lower levels to extend battery life. The prior 5 volt "standard" has been replaced by a 3.3 volt standard, and this may soon be replaced by an even lower 2.7 volt standard. As the regulated voltage level drops, the difficulty in providing an efficiently regulated output voltage in an efficient manner increases, due in part to the increasingly significant effects of the voltage drops across traditional components (e.g., the 0.4 to 0.7 voltage drop across a diode).
Voltage regulators have been implemented using various techniques. In a switching regulator, the flow of power to a load is regulated by controlling the on and off duty-cycle of one or more power switches coupled to the load (the switches may be coupled in series or in parallel).
Switching regulators are typically classified into different configurations or "topologies." One topology--the single-ended inductor circuit--consists of relatively simple circuits where a switch determines whether the voltage applied to an inductor is V.sub.IN or zero. In this manner, the output voltage is a function of the average voltage applied to the inductor.
The output voltage varies depending on the configuration of individual components and the class of single-ended inductor circuit. For example, placing the switch in series between the input and the output causes the output voltage to be less than the input voltage. Such a circuit is commonly referred to as a "step-down" or "buck" converter. By placing the switch in parallel with the output, the output voltage can be made to be greater than the input voltage. Such a circuit is commonly referred to as a "step-up" or "boost" converter. Buck and boost circuits may also be combined--as a "buck-boost" regulator--to provide an inverted-polarity output. One disadvantage of these circuits is that they are limited to a single regulated output. When multiple outputs are required, multiple regulators are used which require multiple feedback or control loops for proper operation, thereby increasing part count, complexity and cost.
Another regulator topology is the transformer-coupled switching regulator. This topology provides an increased ability to achieve current or voltage gain, as well as the possibility of isolation between input and output provided by the transformer. As was the case with single-ended inductor regulators, transformer-coupled regulators are also grouped into classes. So-called "flyback converters" use a transformer to transfer energy from input to output. By adjusting the turns ratio (N) between the primary and secondary of the transformer, the regulator may be caused to provide output voltages which are higher or lower than the input voltage. One disadvantage of flyback converters is the high energy which must be stored in the transformer for proper operation. This requires relatively large magnetic cores and may reduce overall circuit efficiency.
The "forward converter" is another class of transformer-coupled regulator. In the forward converter, an additional winding is provided along with other components, such as diodes and capacitors, which essentially eliminates the problem of large stored energy in the transformer core. The additional (or reset) winding normally has a 1:1 turns ratio to the primary winding to help define the switch voltage when the primary switch is turned off. Unfortunately, the turns ratio may limit the duty-cycle of the device to 50% maximum, above which the switch current will typically rise in an uncontrolled manner. If the number of turns on the reset winding is reduced to increase duty-cycle, there is typically a corresponding increase in the level of switch voltage.
It is also possible to provide other combined configurations, such as a "buck-flyback hybrid" converter, in order to attempt to utilize the best properties of different topologies. Previously known buck-flyback converters provide multiple regulated outputs by generating a main output (e.g., 3.3 volts) using a buck converter and adding an auxiliary winding which operates as a flyback converter for a secondary output (e.g., 5 volts). One disadvantage of these converters is the fact that the output power of the auxiliary winding is limited by the output power of the main output. Additionally, when the auxiliary output is heavily loaded, the buck inductor will observe increased output voltage ripple due to the transformer effects of the auxiliary winding. These, as well as other, disadvantages severely limit the input and output operational ranges of these devices. Also, the flyback action of this converter may result in a severely reduced overall efficiency.
One method that has been utilized heretofore to achieve increased operational efficiency of voltage regulators employs synchronous rectification. In synchronous rectification, a pair of switches, which are connected in series between the input voltage and ground, are synchronized so that either the input voltage or ground is applied to the input of the inductor. The synchronous control of the switches provides a great improvement in efficiency as compared to the traditional circuits which employed a switch and a diode. One disadvantage of these synchronous circuits, however, is the fact that they are unitary in nature--one feedback/control loop is needed for each regulated output. Therefore, a regulator supplying 5 volt and 3.3 volt outputs requires two control loops.
In view of the foregoing, it would be desirable to provide a voltage regulator having multiple regulated outputs, at user selectable voltage levels, using a single control loop.
It would also be desirable to provide a voltage regulator having multiple regulated outputs which operates over a broad range of input voltages and output power requirements.
It would be additionally desireable to provide a voltage regulator having multiple outputs which can provide full power to any one regulated output regardless of the loading on the other outputs.